1. Field of the Invention
The invention relates in general to superconducting induction apparatus, and in particular to means for reducing the operational losses in superconducting windings.
2. Description of the Prior Art
Applications for superconducting windings have been limited because of their AC losses and because of their intolerance to transients. These transients may correspond to variations in the magnetic field the superconducting winding is operating in, or they may be variations in the current it carries. Power distribution systems are particularly vulnerable to fault or short-circuit transient currents.
An inherent characteristic of superconducting wire is that at some critical value of current density and/or magnetic field, the superconducting wire will go normal, i.e., become resistant. If a superconducting winding is allowed to go normal, such as due to a current or field transient which exceeds a critical value, a large amount of heat will be generated. The heat generated may cause the cryogenic fluid cooling the superconductor to evaporate at an explosive rate and/or burn out the coil. Further, the work which must be done on the cryogenic fluid to remove the heat generated in the superconductor is on the order of 500 times the actual amount of energy to be removed from the system.
Thus, on the one hand the system should be small enough to keep operational losses to a minimum, and on the other hand it should be large enough to ensure there will be sufficient capacity to minimize the above-mentioned undesirable consequences when the system is subjected to a typical fault current or other transient.
Copper cladding of the individual filaments has been employed in the prior art to stabilize the superconducting wire against normalization of small regions due to temperature fluctuations, by providing an additional path for the current when the superconducting critical current density is exceeded and the superconductor starts to go normal. This allows the current to flow in the copper layer during a short period of time until the wire is again cooled below the critical point.
For large current transients, the technique employed in the prior art is to design the superconducting winding with the capacity to carry a typical fault current, i.e., size the superconducting wire approximately 10 times that which is necessary to carry the normal operating currents. However, the hysteresis loss in a superconducting wire that is subject to a time varying magnetic field is proportional to its volume under typical conditions of interest here. A modern large transformer is highly efficient, and the resistive losses of the windings are considerably under 1/2% of the total power transferred. Thus, there would be no advantage in reducing the resistive losses associated with windings of a conventional transformer, and replacing them with AC losses, i.e., hysteresis losses of a superconducting winding.
To reduce the amount of superconductor material in a winding which will still carry a fault current it is known to connect a copper auxiliary winding in parallel with a superconducting winding. In fact, copper cladding of the superconducting wire, referred to above, is a form of an auxiliary copper winding, i.e., it is an additional path to transfer current from a superconducting winding. However, this method, like the usual all-superconducting winding, has detrimental aspects which offset the possible benefits. Copper windings are bulky. The current density capability of a superconducting winding is one thousand to ten thousand times the current density capability of a copper winding. Since it is the auxiliary winding which must be sized to carry a fault current on the order of ten times the rated current, the copper auxiliary winding then would have almost the same physical size as a traditional copper winding of the standard power transformer. The potential size and weight reduction of the superconducting winding would be lost. Another detrimental effect of the copper auxiliary is that now the transformer system will experience resistive losses whenever there is current flow in the auxiliary, which can occur for prolonged periods during overload conditions.
Accordingly, it would be desirable to minimize the operational losses in a superconducting winding by reducing the volume of the superconducting coil that is subject to the time varying magnetic field, and thereby subject to hysteresis losses. It would further be desirable to provide means for carrying an overload current, which is not subject to large hysteresis losses during normal operation.